For some reason and for many years there's a limit for font size in Illustrator. It is 1296 points max, so you can't go bigger without converting type to outlines first. Converting to outlines means you will not be able to edit type parameters and text it-self. Don't know how about you, but this makes me sad in non-destructive editing era.

Why do you want text bigger than 1296pt? That is very big. If you are designing for any format large enough it's normal to design at smaller size or If you are using the text shapes as some background or design element then outlining shouldn't be a problem.
– Cai♦Dec 25 '15 at 21:15

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Well, let's make a letter sign 50 cm high, that's not a big sign, right? So, 50 cm high letters with Avant Garde font are equal to 1870 pt. Pretty narrow font-size limit, I would say. Yes, you can design at smaller size, then convert type to outlines and scale it up, but this is just another workaround with its own drawbacks. You'll need to convert all type objects to outlines and resize the design each time you asked to make edits, and if you design a poster and think in the process "is this QR code large enough" each time you have to multiply in mind for your shrink value to get actual size.
– Ignat MillerDec 25 '15 at 22:43

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Also I am the person who LOVES non-destructive editing, and I don't like to convert something to outlines just to scale it up a bit more - that's nonsense imo.
– Ignat MillerDec 25 '15 at 22:47

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Working with scaled down design just because the font size I need is higher than 35 cm seems to me like a narrow limits, not a big demands.
– Ignat MillerDec 25 '15 at 22:55

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When I say work at a smaller size, what i mean is, say your designing a banner that is 3m x 1m and will be printed at 150dpi, you design at 1.5m x 0.5m at 300dpi. It's common practice and I don't really see the problem with it.
– Cai♦Dec 25 '15 at 23:06

Now you can scale up text object to any size and keep it fully editable while font size is still 1296 pt. That's a good news, but the problem is that the text object it-self is still the same size it only appears bigger and if you'll hover the mouse pointer over it you'll see the actual size. Also if you'll switch to Outline view (Ctrl+Y) text object will appear without Transform... effect, so editing in Outline view becomes a guess work. And if you'll try to convert type to outlines you'll get actual 1296 pt size type outlined not the one you see. And that's where you can do the last try to fight the limits.

Go to Object > Expand Appearance

Now bounding box will become adequate to the text object size, text and type properties will still be fully editable, moreover now in Outline view you'll see the object as you see it in Preview. And even outlining type will now create outlines from what you see. Woah! Perfect workaround? Not really...

Very annoying drawback of this workaround is that each time you try to transform (move, rotate, reflect, scale, shear or align) the text object enlarged with Transform... effect, the effect immediately evaporates and text object become 1296 points size again.

How you can deal with it:

Expand Appearance only after you've made all necessary adjustments.

Use Transform... effect again to adjust already expanded object in case you only need to move, scale or rotate your text object. Don't forget to Expand Appearance once again after applying the effect.

You can down-scale the text object by decreasing font size down from 1296 pt. Also you can initially set font size to 1000 pt, roughly scale up text object with Transform... effect, apply effect and then fine-tune scaling by adjusting font size.

And here's another obvious option: as Cai mentioned, you can simply down-scale your whole document by 2 (or 3, or any number) and then increase document`s dpi by the same number. Cai gives an example: if your desirable document size is 3m x 1m at 150 dpi you can down-scale it to 1.5m x 0.5m, but use 300 dpi instead. Don't forget to mention this willful size mismatch if you share your project files with others.

A simple answer to your question is that you can create outlines from your text that are infinitely scaleable by pressing cmd+shift+o on a mac or ctrl+shift+o on a PC while your text is selected.

The real question is why are you needing a font that large in illustrator? You can design for billboards on a canvas that is only in the hundreds of points and still have a clear image. If you are dealing with images as well as vectors, I recommend using InDesign or Photoshop. That's what those were made for.